Microorganism Producing L-Methionine Precursor and Method of Producing L-Methionine and Organic Acid from the L-Methionine Precursor

ABSTRACT

The present invention relates to a method for producing L-methionine, comprising: i) culturing an L-methionine precursor-producing microorganism strain in a fermentation solution, so that the L-methionine precursor accumulates in the solution; and ii) mixing a converting enzyme and methylmercaptan or its salts with at least a portion of the solution to convert the accumulated L-methionine precursor into L-methionine, as well as to microorganism strains used in each step.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser. No. 13/752,907, filed Jan. 29, 2013, which is a continuation application of U.S. application Ser. No. 12/066,111, filed Mar. 7, 2008, which is a 35 U.S.C. §371 national phase application of PCT/KR2007/003650 (WO 2008/013432), filed on Jul. 30, 2007, each entitled “Microorganism producing L-methionine precursor and method of producing L-methionine and organic acid from the L-methionine precursor,” which application claims the benefit of Korean Application No. 10-2007-0076045, filed Jun. 27, 2007 and Korean Application No. 10-2006-0071581, filed Jul. 28, 2006, each of which is incorporated herein by reference in its entirety.

SEQUENCE LISTING

The instant application contains a Sequence Listing which has been submitted in ASCII format via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Apr. 7, 2015, is named Sequence_List_Revised.txt and is 88.8 KB in size.

TECHNICAL FIELD

The present invention relates to a method for producing L-methionine and organic acid. More particularly, the present invention relates to a method for producing L-methionine and organic acid with high yield by enzyme conversion reaction from L-methionine precursor produced by the fermentation of L-methionine precursor-producing strain prepared according to the present invention. The method of the present invention to produce L-methionine is more pro-environmental than the conventional method and enables selective production of L-methionine so as to use L-methionine in various fields of industry as feed, food additives and a raw material for medical supplies and drugs, etc.

BACKGROUND ART

Methionine is one of essential amino acids of human body which has been widely used as feed and food additives and further used as a synthetic raw material for medical solutions and medical supplies. Methionine acts as a precursor of such compounds as choline (lecithin) and creatine and at the same time is used as a synthetic raw material for cysteine and taurine. Methionine can also provide sulfur. S-adenosyl-methionine is derived from L-methionine and plays a certain role in providing methyl group in human body and also is involved in the synthesis of various neurotransmitters in the brain. Methionine and/or S-adenosyl-L-methionine (SAM) inhibits fat accumulation in the liver and artery and alleviates depression, inflammation, liver disease, and muscle pain, etc.

The in vivo functions of methionine and/or S-adenosyl-L-methionine known so far are as follows.

1) It inhibits fat accumulation in the liver and artery promoting lipid metabolism and improves blood circulation in the brain, heart and kidney(J Hepatol. Jeon B R et al., 2001 Mar; 34(3): 395-401).

2) It promotes digestion, detoxification and excretion of toxic substances and excretion of heavy metals such as Pb.

3) It can be administered as an anti-depression agent at the dosage of 800-1,600 mg/day (Am J Clin Nutr. Mischoulon D. et al., 2002 November; 76(5): 1158S-61S).

4) It enhances liver functions (FASEB J. Mato J M., 2002 January; 16(1): 15-26) and particularly is effective in the liver disease caused by alcohol (Cochrane Database Syst Rev., Rambaldi A., 2001; (4): CD002235).

5) It has anti-inflammatory effect on bone and joint diseases and promotes joint-recovery (ACP J Club. Sander O., 2003 January-February; 138(1): 21, J Fam Pract., Soeken K L et al., 2002 May; 51(5): 425-30).

6) It is an essential nutrient for hair. It provides nutrition to hair and thereby prevents hair loss (Audiol Neurootol., Lockwood D S et al., 2000 September-October; 5(5): 263-266).

Methionine can be chemically or biologically synthesized to be applied to feed, food and medicines.

In the chemical synthesis, methionine is mostly produced by hydrolysis of 5-(β-methylmercaptoethyl)-hydantoin. The chemically synthesized methionine has a disadvantage of only being produced as a mixed form of L-type and D-type.

In the biological synthesis, methionine is produced by method using proteins involved in methionine synthesis. L-methionine is biosynthesized from homoserine by the action of the enzyme expressed by such genes as metA, metB, metC, metE and metH. Particularly, metA is the gene encoding homoserine O-succinyl transferase which is the first enzyme necessary for methionine biosynthesis, and it converts homoserine into O-succinyl-L-homoserine. O-succinylhomoserine lyase or cystathionine gamma synthase coded by metB gene converts O-succinyl-L-homoserine into cystathionine. Cystathionine beta lyase coded by metC gene converts cystathionine into L-homocysteine. MetE encodes cobalamine-independent methionine synthase and metH encodes cobalamine-dependent methionine synthase, both of which convert L-homocysteine into L-methionine. At this time, 5,10-methylenetetrahydrofolate reductase coded by metF and serine hydroxymethyltransferase coded by glyA work together to synthesize N(5)-methyltetrahydrofolate providing methyl group necessary for L-methionine synthesis.

L-methionine is synthesized by a series of organic reactions by the above enzymes. The genetic modification on the above proteins or other proteins affecting the above proteins might result in the regulation of L-methionine synthesis. For example, Japanese Laid-Open Patent Publication No. 2000/139471 describes a method of producing L-methionine with the Escherichia sp. of which thrBC and metJ genes on the genome are deleted, metBL is over-expressed and metK is replaced by a leaky mutant. Also, US Patent Publication No. US2003/0092026 A1 describes a method using a metD (L-methionine synthesis inhibitor) knock-out microorganism which belongs to Corynebacterium sp. US Patent Publication No. US2002/0049305 describes a method to increase L-methionine production by increasing the expression of 5,10-methylenetetrahydrofolate reductase (metF).

US Patent No. US2005/0054060A1 describes the method of preparing L-methionine producing microorganism using cystathionine synthase (O-succinylhomoserine lyase) mutant. This cystathionine synthase mutant can produce homocysteine or methionine directly from H₂S or CH₃SH instead of cysteine. In this method, mutant cystathionine synthase is directly introduced into a cell and participated in the intracellular methionine biosynthesis procedure. In this method, cystathionine synthase reaction is not very efficient due to the use of intracellular methionine biosynthesis pathway. Also, the high toxicity of H₂S or CH₃SH to the cells reduces the effectiveness of this method. In the experiment, it was also found that the substrate specificity of cystathionine synthase to CH₃SH is very low compared to succinylhomoserine lyase derived from Pseudomonas or Chromobacterium sp.

According to the previous reports, cystathionine synthase tend to produce various products by reaction with various substrates. Cystathionine synthase mediates the interaction between homocysteine and O-succinyl homoserine to produce homolanthionine with high efficiency (J. Bacteriol (2006) vol 188:p 609-618). The cystathionine synthase in a cell can interact with various methionine precursors and can produce various byproducts with high efficiency. Therefore, overexpression of Cystathionine synthase can make lower the reaction efficiency due to the higher production of byproduct.

The methionine produced by the conventional biological method is L-type, which has advantages but the production amount is too small. This is because the methionine biosynthetic pathway has very tight feed-back regulation systems. Once methionine is synthesized to a certain level, the final product methionine inhibits the transcription of metA gene encoding the primary protein for initiation of methionine biosynthesis. Over-expression of metA gene itself cannot increase methionine production because the metA gene is suppressed by methionine in the transcription stage and then degraded by the intracellular proteases in the translation stage (Dvora Biran, Eyal Gur, Leora Gollan and Eliora Z. Ron: Control of methionine biosynthesis in Escherichia coli by proteolysis: Molecular Microbiology (2000) 37(6), 1436-1443). Therefore, many of previous patents were focused on how to free the metA gene from its feed-back regulation system (WO2005/108561, WO1403813).

When methionine is produced in biological system, produced methionine is converted to S-adenosylmethionine by S-adenosylmethionine synthase in the methionine degradation pathway. S-adenosylmethionine synthase cannot be deleted because S-adenosylmethionine is an essential substance in cells. According to the previous patents, the gene encoding S-adenosylmethionine synthase was mutated to have low activity to increase the methionine production (WO2005/108561).

DISCLOSURE OF THE INVENTION

The conventional methionine biosynthesis method uses cystathionine synthase metabolism pathway to produce methionine, so the enzyme reaction process is inefficient due to the sulfide toxicity and byproducts generation. In addition, feed-back regulation in methionine synthesis pathway inhibits mass-production of methionine.

It is an object of the present invention to provide an alternative method of producing L-methionine to overcome the above problems of the conventional method. The alternative method is composed of two-step process in which L-methionine precursor is produced by fermentation and L-methionine precursor is converted to L-methionine by enzymes.

It is another object of the present invention to provide a method for producing L-methionine selectively.

It is further an object of the present invention to provide a method for simultaneously producing organic acid as a byproduct without an additional process.

The present invention is described in detail hereinafter.

To achieve the object of the invention, the present invention provides a method for producing L-methionine comprising the steps of 1) preparing L-methionine precursor producing strain and producing L-methionine precursor by the fermentation of the strain, and 2) producing L-methionine and organic acid by the enzyme reaction with the L-methionine precursor.

Particularly, in step 1) process, an L-methionine precursor producing strain is generated and fermented for the accumulation of L-methionine precursor in the culture media. At this time, the strain for the production of L-methionine precursor is prepared by the method of the invention designed by the inventors, so this invention also includes the strain and the method for generating the strain in its scope.

The L-methionine precursor herein is represented by one of the O-acyl homoserine group composed of the following formula;

Wherein

R is a substance including C, H, O, N and other compounds with 15 carbon molecules at maximum. For example, the O-acyl homoserine group includes, but not limited to, O-acetyl homoserine, O-succinyl homoserine, propionyl homoserine, acetoacetyl homoserine, coumaroyl homoserine, malonyl homoserine, hydroxymethylglutaryl homoserine and pimelylhomoserine.

The L-methionine precursor of the present invention is preferably O-acetyl homoserine or O-succinyl homoserine.

The “L-methionine precursor-producing strain” as used herein refers to a prokaryotic or eukaryotic microorganism strain that is able to accumulate L-methionine precursor by the manipulation according to the present invention. For example, the strain can be selected from the group consisting of Escherichia sp., Erwinia sp., Serratia sp., Providencia sp., Corynebacteria sp., Pseudomonas sp., Leptospira sp., Salmonellar sp., Brevibacteria sp., Hyphomonas sp., Chromobacterium sp. and Nocardia sp. microorganisms or fungi or yeasts. Preferably, the microorganisms of Pseudomonas sp., Nocardia sp. and Escherichia sp. can be used to produce O-succinylhomoserine, and the microorganisms of Escherichia sp., Corynebacterium sp., Leptospira sp. and yeasts can be used to produce O-acetylhomoserine. More preferably, the microorganisms of Escherichia sp. can be used, and most preferably Escherichia coli (hereinafter referred to as “E. Coli”) can be used. In addition, the foreign genes can be introduced into the Escherichia sp. microorganism to selectively produce O-succinyl homoserine and O-acetyl homoserine.

The present invention provides an L-methionine precursor-producing strain in which the genes involved in the degradation of O-succinyl homoserine or O-acetyl homoserine is deleted or weakened. The present invention also provides an L-methionine precursor-producing strain in which the genes involved in the synthesis of O-succinyl homoserine or O-acetyl homoserine is introduced or enhanced. The present invention also selectively provides a strain in which threonine biosynthesis pathway is blocked or weakened to enhance O-succinyl homoserine or O-acetyl homoserine production. The present invention further provides a strain in which the genes which are free from feedback regulation system and encoding the proteins involved in the synthesis of O-succinyl homoserine or O-acetyl homoserine are introduced, over-expressed or activity-enhanced.

More particularly, the present invention provides an L-methionine precursor producing strain by deleting metB gene involved in the degradation of L-methionine precursor, thrB gene involved in threonine biosynthesis pathway and metJ gene repressing the transcription of L-methionine precursor synthesis gene and by enhancing the expression of the metA or metX gene involved in L-methionine precursor biosynthesis or introducing the metA or metX gene free from feed-back regulation system; or knocking-out metA gene and instead introducing metX gene; or deleting metX gene and instead introducing metA gene.

In the present invention, a deletion of the gene can be performed by cutting out of a region of the gene or modifying the protein sequence by introducing a specific DNA sequence on the chromosome. The weakening of the gene can be performed by reducing the protein activity by introducing the mutation in the ORF region of the target gene or by reducing the protein expression by the modification of a promoter region or of 5′-UTR nucleotide sequence of the gene.

In the present invention, the enhancement of the protein expression can be performed by the modification of the promoter region of the gene or the nucleotide sequence of the 5′-UTR region, and the enhancement of the activity of the protein can be performed by introducing the mutation in the ORF region of the target gene, and enhancement of the protein expression can also be performed by the introduction of the extra copy of target gene on the chromosome end or by the introduction of the vector harboring the target gene with the self-promoter or enhanced other promoter in the strain.

In a preferred embodiment of the present invention, the method for preparing an L-methionine precursor producing strain is as follows;

In step 1, a gene encoding such proteins as cystathionine gamma synthase, O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase is deleted or weakened in a strain in order to accumulate L-methionine precursor such as O-succinyl homoserine or O-acetyl homoserine.

The gene encoding cystathionine gamma synthase is indicated as metB, the gene encoding O-succinylhomoserine sulfhydrylase is indicated as metZ, and the gene encoding O-acetylhomoserine sulfhydrylase is indicated as metY. A gene encoding the protein having the above mentioned activity is exemplified by metB which was known for E. Coli. The genomic sequence of the gene can be obtained from the genomic sequence of E. coli (Accession no. AAC75876) informed in the previous report (Blattner et. al., Science 277: 1453-1462 (1997)). The above genomic sequence also can be obtained from NCBI (National Center for Biotechnology Information) and DDBJ (DNA Data Bank Japan). Other genes having the above activity are exemplified by metB and metY derived from Corynebacterium, and metZ derived from Pseudomonas.

Cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase has the activity to convert O-succinyl homoserine or O-acetylhomoserine into cystathionine or homocysteine as shown in the following reaction formulas. Therefore, the strain in which the genes having these activities are deleted or weakened, showed the accumulation of O-succinylhomoserine or O-acetylhomoserine in the culture solution.

L-cysteine+O-succinyl-L-homoserine<=>succinate+cystathionine

L-cysteine+O-acetyl-L-homoserine<=>acetate+cystathionine

HS⁻+O-succinyl-L-homoserine<=>succinate+homocysteine

HS⁻+O-acetyl-L-homoserine<=>acetate+homocysteine

In step 2, thrB gene encoding homoserine kinase in the strain prepared in step 1 is deleted or weakened. The thrB gene is involved in the synthesis of O-phosphohomoserine from homoserine, which is then converted into threonine by thrC gene. The thrB gene is deleted or weakened to use all the produced homoserine for the synthesis of methionine precursor.

In step 3, the metJ gene, the transcription regulator of metA gene, is deleted or weakened. The metA gene involved in the synthesis of methionine precursor is regulated by feed-back regulation system of methionine and the metJ gene is a repressor involved in the transcription of metA gene. To over-express the metA gene constitutively and activate the synthesis methionine precursor, the elimination of the metA gene transcription repressor is profitable. Therefore, the metJ gene is eliminated in E. coli and the metA gene expression is increased, which can lead (to) the mass-production of L-methionine precursor.

The above steps 2 and 3 can be modified according to a precursor producing strain and might not be necessary for the precursor producing strain. However, it can be more preferably executed to enhance the precursor production pathway in the microorganism of Escherichia sp.

In step 4, the expression of metA or metX gene encoding homoserine O-succinyl transferase or homoserine O-acetyl transferase which is the enzyme mediating the first stage of methionine biosynthesis pathway is enhanced to promote the methionine precursor synthesis. The metA gene is the general term for the gene encoding homoserine O-succinyl transferase, and the metX gene is the general term for the gene encoding homoserine O-acetyl transferase. To enhance the expression of metA or metX gene, an additional copy of gene can be introduced or 5′-UTR or a promoter can be modified or ORF of each gene can be mutated. The enhancement of expression of this gene results in the significant increase of the L-methionine precursor synthesis.

If methionine is considered to be necessary for the growth of a strain, metA or metX gene free from feed-back regulation can be introduced. In this case, L-methionine precursor can be synthesized regardless of methionine content in the medium and so the addition of methionine to the medium facilitates the synthesis of L-methionine precursor and the growth of the cells.

To increase O-acetylhomoserine production from the O-succinylhomoserine producing strain, metA gene encoding homoserine O-succinyl transferase existing in the chromosome can be deleted. Where the production of O-succinylhomoserine is inhibited by deletion of metA gene and O-acetylhomoserine is produced by additionally introducing metX gene, O-acetylhomoserine can be produced with higher yield compare with the case of introducing metX gene in the presence of the metA gene.

It is also possible to increase O-succinylhomoserine production in O-acetylhomoserine producing strain by deleting metX gene encoding homoserine O-acetyl transferase existing in the chromosome of the strain. Where the production of O-acetylhomoserine is inhibited by deletion of metX gene and O-succinylhomoserine is produced by additionally introducing metA gene, O-succinylhomoserine can be produced with higher yield.

O-succinylhomoserine or O-acetylhomoserine, L-methionine precursor, can be accumulated in a strain by taking advantage of a part or the entire process of the above step 1-step 4.

The L-methionine precursor producing strain can be prepared from the strain producing L-lysine, L-threonine or L-isoleucine. Preferably, it can be prepared by using the L-threonine producing strain. With this strain, homoserine synthesis is already higher and the production of methionine precursor can be resultantly increased. So, methionine precursor can be accumulated by deleting or weakening a gene involved in threonine biosynthesis pathway and then metA or metY or MetZ gene, using the L-threonine producing strain. It is more preferred to delete or weaken thrB gene first and then metB, metY or metZ to synthesize methionine precursor. In the meantime, the enhancement of metA or metX gene expression results in the increase of methionine precursor synthesis.

The “L-threonine-producing strain” of the invention refers to a prokaryotic or eukaryotic microorganism strain that is able to produce L-threonine in vivo. For example, the strain can be include L-threonine producing microorganism strains belongs to Escherichia sp., Erwinia sp., Serratia sp., Providencia sp., Corynebacterium sp. and Brevibacterium sp. Among these, Escherichia sp. microorganism is preferred and Escherichia coli is more preferred.

The L-threonine producing strain includes not only the microorganisms in nature but also their mutants which are exemplified by microorganisms that has a leaky requirement for isoleucine and is resistant to L-lysine analogues and α-aminobutyric acid; and is mutated by additionally introducing at least an extra copy of endogenous phosphoenol pyruvate carboxylase(ppc) gene; and is inactivated pckA gene involved in the conversion process of oxaloacetate(OAA) that is an intermediate of L-methionine synthesis into phosphoenol pyruvate(PEP); and is inactivated tyrR gene inhibiting the expression of tyrB gene involved in L-methionine biosynthesis; and is inactivated galR gene inhibiting the expression of galP gene involved in glucose transport. The L-lysine analogs herein may be one or more compounds selected from the group consisting of S-(2-aminoethyl)-L-cysteine and δ-methyl-L-lysine.

In a preferred embodiment of the present invention, CJM002, the L-threonine producing and L-methionine-independent strain mutated from TF4076(KFCC 10718, Korean Patent No. 92-8365), the L-threonine producing E. coli mutant strain, was used. TF4076 has a requirement for methionine, and is resistant to methionine analogues (ex, α-amino-β-hydroxy valeric acid, AHV), lysine analogues (ex, S-(2-aminoethyl)-L-cysteine, AEC), and isoleucine analogues (ex, α-aminobutylic acid). The general information contained in the above Korean Patent can be included in the scope of the present invention by claims. The TF4076 is not able to synthesize methionine in vivo because it is the strain which has a requirement for methionine. To use this strain as the methionine producing strain of the invention by free from a requirement for methionine, the present inventors prepared the L-threonine producing strain E. Coli CJM002 free from the requirement for methionine by artificial mutation using NTG. The E. coli CJM002 was named as Escherichia coli MF001 and deposited at KCCM (Korean Culture Center of Microorganism, Eulim Buld., Hongje-1-Dong, Seodaemun-Ku, Seoul, 361-221, Korea) on Apr. 9, 2004 (Accession No: KCCM-10568). The O-succinylhomoserine producing Escherichia coli CJM-BTJ (pMetA-CL) prepared by the above method was also deposited on Jul. 21, 2006 (Accession No: KCCM-10767) and Escherichia coli CJM-BTJ (pCJ-MetA-CL) was deposited on Jul. 5, 2007 (Accession No: KCCM-10872). The O-acetylhomoserine producing Escherichia coli CJM-BTJA (pCJ-MetX-CL) prepared by the above method of the invention was also deposited on Jul. 5, 2007 (Accession No: KCCM-10873).

The culture of the L-methionine precursor producing strain prepared above can be performed by a proper medium and conditions known to those in the art. It is well understood by those in the art that the culture method can be used by easily adjusting, according to the selected strain. For example, he culture method including, but not limited to batch, continuous culture and fed-batch. A variety of culture methods are described in the following reference: “Biochemical Engineering” by James M. Lee, Prentice-Hall International Editions, pp 138-176.

The medium has to meet the culture conditions for a specific strain. A variety of microorganism culture mediums are described in the following reference: “Manual of Methods for General Bacteriology” by the American Society for Bacteriology, Washington D.C., USA, 1981. Those mediums include various carbon sources, nitrogen sources and trace elements. The carbon source is exemplified by carbohydrate such as glucose, sucrose, lactose, fructose, maltose, starch, cellulose; fat such as soybean oil, sunflower oil, castor oil and coconut oil; fatty acid such as palmitic acid, stearic acid, and linoleic acid; alcohol such as glycerol and ethanol; and organic acid such as acetic acid. One of these compounds or a mixture thereof can be used as a carbon source. The nitrogen source is exemplified by such organic nitrogen source as peptone, yeast extract, gravy, malt extract, corn steep liquor (CSL) and bean flour and such inorganic nitrogen source as urea, ammonium sulfate, ammonium chloride, ammonium phosphate, ammonium carbonate and ammonium nitrate. One of these compounds or a mixture thereof can be used as a nitrogen source. The medium herein can additionally include potassium dihydrogen phosphate, dipotassium hydrogen phosphate and corresponding sodium-containing salts as a phosphate source. The medium also can include a metal salt such as magnesium sulfate or iron sulfate. In addition, amino acids, vitamins and proper precursors can be added as well. The mediums or the precursors can be added to the culture by batch-type or continuously.

pH of the culture can be adjusted during the cultivation by adding in the proper way such a compound as ammonium hydroxide, potassium hydroxide, ammonia, phosphoric acid and sulfuric acid. The generation of air bubbles can be inhibited during the cultivation by using an antifoaming agent such as fatty acid polyglycol ester. To maintain aerobic condition of the culture, oxygen or oxygen-containing gas (ex, air) can be injected into the culture. The temperature of the culture is conventionally 20-45° C., preferably 25-40° C. The period of cultivation can be continued until the production of L-methionine precursor reaches a wanted level, and the preferable cultivation time is 10-160 hours.

Step 2) process includes the process for producing L-methionine and organic acid by enzyme reaction using an enzyme having the activity of cystathionine synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase or the strain containing these enzyme activities by using O-succinylhomoserine or O-acetylhomoserine produced from the above L-methionine precursor producing strain and methylmercaptan as a substrate.

More particularly, the present invention provides the method for producing L-methionine by enzyme reaction of cystathionine synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase by using homoserine, O-phosphohomoserine, O-succinyl homoserine or O-acetyl homoserine accumulated from the above method as a substrate. It is preferred in the present invention to use O-succinylhomoserine or O-acetylhomoserine as a substrate.

In the present invention, the cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase can be derived from Escherichia Sp., Pseudomonas sp., Leptospira sp., Corynebacterium sp., Saccharomyces sp., Chromobacterium sp., Nocardia sp., Bradyrhizobium sp., Hyphomonas sp., Methylococcus sp., Methylobacillus sp., Nitrosomonas sp., Klebsiella sp., Bacillus sp., Shigella sp., Colwellia sp., Salmonella sp., yeast, or fungi.

In step 2) process, where O-succinylhomoserine is used as an L-methionine precursor, preferably cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase derived from Pseudomonas sp., Nocardia sp. or Chromobacterium sp., more preferably derived from Pseudomonas aurogenosa, Nocardia Farcinica, Pseudomonas putida or Chromobacterium Violaceum can be used.

In step 2) process, where O-acetylhomoserine is used as an L-methionine precursor, preferably cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase derived from Leptospira sp., Chromobacterium sp., or Hyphomonas sp., more preferably derived from Leptospira meyeri, Pseudomonas aurogenosa, Hyphomonas Neptunium or Chromobacterium Violaceum can be used.

The enzyme reactions above are as shown in the following reaction formulas and the structural formulas are shown in FIG. 2.

CH₃SH+O-succinyl-L-homoserine<=>succinate+methionine

CH₃SH+O-acetyl-L-homoserine<=>acetate+methionine

In the above reactions, as shown in formulas of FIG. 2, CH₃S— residue of methylmercaptan is substituted with succinate or acetate residue of O-succinylhomoserine or O-acetylhomoserine to produce methionine. Methylmercaptan (CH₃SH) can be added in different forms during the reaction.

The sequence of the genes encoding the enzymes having the above enzyme activity can be obtained from the database of NCBI, USA, and DNA data bank (KEGG), Japan.

For the biological conversion reaction, a gene is cloned from the obtained gene sequence, which is then introduced into an expression vector. The enzyme is expressed in active form from a recombinant strain. Both the enzyme expressing strain and the expressed enzyme can be directly used for the reaction.

The enzymes expressed from above genes or the microbial strains expressing those enzymes can be directly mixed, partly or not, with the fermentation supernatant or the fermentation broth accumulated with L-methionine precursor to start the reaction. In a preferred embodiment of the invention, O-succinylhomoserine or O-acetylhomoserine accumulated in the fermentation solution can be converted into methionine by cystathionine gamma synthase or O-acetylhomoserine sulfhydrylase or O-succinylhomoserine sulfhydrylase derived from Pseudomonas sp., Chromobacterium sp., Leptospira sp. or Hyphomonas sp..

More preferably, O-succinylhomoserine accumulated in the fermentation solution is converted into methionine by cystathionine gamma synthase or O-acetylhomoserine sulfhydrylase or O-succinylhomoserine sulfhydrylase derived from Pseudomonas aurogenosa, Pseudomonas putida or Chromobacterium Violaceum.

O-acetylhomoserine accumulated in the fermentation solution is converted into methionine by cystathionine gamma synthase or O-acetylhomoserine sulfhydrylase or O-succinylhomoserine sulfhydrylase derived from Leptospira meyeri, Hyphomonas Neptunium or Chromobacterium Violaceum.

Each gene was expressed in pCL-CJ1 vector (CJ, Korea), the expression vector for E. coli, and the expressed protein was obtained from enzyme solution prepared by cell lysis using sonication. The enzyme solution was added to the fermentation solution accumulated O-succinylhomoserine or O-acetylhomoserine, and methylmercaptan solution was also added thereto to start the reaction. The reaction was confirmed using DTNB (5,5-dithiobis(2-nitro-benzoic acid, Sigma, USA) and the reaction product was analyzed by HPLC.

In the present invention, byproducts such as succinic acid or acetic acid can be additionally obtained, without a separate production process, by the reaction of CH3SH with O-succinylhomoserine and O-acetylhomoserine respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

The application of the preferred embodiments of the present invention is best understood with reference to the accompanying drawings, wherein:

FIG. 1 is a diagram illustrating genetic manipulation of the methionine precursor producing strain.

FIG. 2 is a diagram illustrating chemical structures of 2-step process for the production of methionine.

FIG. 3 is a schematic diagram of pMetA-CL for the expression of metA gene.

FIG. 4 is a schematic diagram of pCJ-MetB-CL for the expression of metB gene.

FIG. 5 is a graph showing the reaction curves illustrating the O-succinylhomoserine consumptions by various enzymes.

The origin of each enzyme solution is as follows. Enzyme solution #21 is a cell extract not containing a specific gene.

Strain Gene Substrate number name specificity Strain (ATCC) (KEGG) OSH OAH Escherichia Coli K12 55151 MetB + + Pseudomonas aurogenosa 17933 MetZ +++ + MetY ++++ ++++ Pseudomonas putida 17390 MetZ ++++ + Corynebacteria glutamicum 13032 MetB + + MetY + + Leptospira meyeri 43278 MetY + ++ Saccharomyces cerevisiae 2704 Met25 + + Chromobacterium Violaceum 12472 MetZ ++++ +++ Nocardia Farcinica 3318 MetZ ++++ + Bradyrhizobium Japonicum 10324 MetZ + + Hyphomonas Neptunium 49408 MetZ + ++++ Methylococcus Capsulatus 19069D-5 MetZ + + Methylobacillus Flagellatus 51484D MetZ + + Nitrosomonas Europaea 19718D MetZ + + Klebsiella Pneumoniae 25955 MetB + + Bacillus Subtilis 10783 MetB + + Shigella flexneri 2457T 700930D-5 MetB + +

FIG. 6 is a graph showing the reaction curves illustrating the O-acetylhomoserine consumptions by various enzymes. Each number is as shown in FIG. 5.

FIG. 7 discloses a diagram illustrating the amino acid sequence of each enzyme used (SEQ ID NOS 58, 56, 60, 59, 77, 61, 78, 62, 57, 79, 55, 80-81, 67, 65, 82, residues 1-371 of 64, 66, 83 and 63, respectively, in order of appearance) for the conversion reaction, arranged by megalign of DNAstar.

FIG. 8 is the continuation of the diagram of FIG. 7, illustrating the amino acid sequence of each enzyme used (SEQ ID NOS 58, 56, 60, 59, 77, 61, 78, 62, 57, 79, 55, 80-81, 67, 65, 82, residues 1-371 of 64, 66, 83 and 63, respectively, in order of appearance) for the conversion reaction, arranged by megalign of DNAstar.

BEST MODE FOR CARRYING OUT THE INVENTION

Practical and presently preferred embodiments of the present invention are illustrative as shown in the following Examples.

However, it will be appreciated that those skilled in the art, on consideration of this disclosure, may make modifications and improvements within the spirit and scope of the present invention.

EXAMPLE 1 Construction of a Methionine Precursor Producing Strain

<1-1> Deletion of metB Gene

To deletion metB gene encoding cystathionine synthase in E. coli strain, FRT-one-step PCR deletion was performed (PNAS (2000) vol 97: P6640-6645). Primers of SEQ. ID. NO: 1 and NO: 2 were used for PCR using pKD3 vector (PNAS (2000) vol 97: P 6640-6645) as a template, resulting in the construction of deletion cassette. PCR was performed as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was electroporated into E. coli (K12) W3110 transformed with pKD46 vector (PNAS (2000) vo197: P6640-6645). Before electroporation, W3110 transformed with pKD46 was cultivated at 30° C. in LB medium containing 100 μg/L of ampicillin and 5 mM of 1-arabinose until OD₆₀₀ reached 0.6. Then, the cultured strain was washed twice with sterilized distilled water and one more time with 10% glycerol. Electroporation was performed at 2500 V. The recovered strain was streaked on LB plate medium containing 25 μg/L of chloramphenicol, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with the same primers as the above under the same condition. The deletion of metB gene was identified by confirming the 1.2 kb sized gene on 1.0% agarose gel. The strain was then transformed with pCP20 vector (PNAS (2000) vol 97: P6640-6645) and cultured in LB medium. The final metB knock-out strain was constructed in which the size of metB gene reduced to 150 by on 1.0% agarose gel by PCR under the same conditions. Chloramphenicol marker was confirmed to be eliminated. The constructed strain was named W3-B.

<1-2> Deletion of thrB Gene

The inventors tried to increase O-succinylhomoserine synthesis from homoserine by deletion of thrB gene encoding homoserine kinase. Particularly, where a threonine producing strain was used, deletion of this gene was quite necessary because the activity of use of homoserine was very strong. To deletion thrB gene in the W3-B strain constructed above, FRT one step PCR deletion was performed by the same manner as described above for the deletion of metB gene.

To construct thrB deletion cassette, PCR was performed by using pKD4 vector (PNAS (2000) vo197: P6640-6645) as a template with primers of SEQ. ID. NO: 3 and NO: 4 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute. The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.6 kbp band. The recovered DNA fragment was electroporated into the W3-B strain transformed with pKD46 vector. The recovered strain was streaked on LB plate medium containing 50 μg/L of kanamycin, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with primers of SEQ. ID. NO: 3 and NO: 4 under the same conditions as the above. The deletion of ThrB gene was identified by selecting the strain whose size is 1.6 kb on 1.0% agarose gel. The strain was then transformed with pCP20 vector and cultured in LB medium. The final thrB knock out strain was constructed in which the size of thrB gene reduced to 150 kb on 1.0% agarose gel by PCR under the same conditions. Kanamycin marker was confirmed to be eliminated. The constructed strain was named W3-BT.

<1-3> Deletion of metJ Gene

To deletion metJ gene which is the regulator gene of metA gene involved in methionine precursor synthesis, FRT one step PCR deletion was performed by the same manner as used for the deletion of metB gene.

To construct metJ deletion cassette, PCR was performed with primers of SEQ. ID. NO: 5 and NO: 6 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was electroporated into the W3-BT strain transformed with pKD46 vector. The recovered strain was streaked on LB plate medium containing chloramphenicol, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with primers of SEQ. ID. NO: 7 and NO: 8 under the same conditions as the above. The deletion of metJ was identified by confirming the 1.6 kb sized gene on the 1.0% agarose gel. The strain was then transformed with pCP20 vector and cultured in LB medium. The final metJ knock out strain was constructed in which the size of metJ gene reduced to 600 kb on 1.0% agarose gel by PCR under the same conditions and the strain Chloramphenicol marker was confirmed to be eliminated. The constructed strain was named W3-BTJ.

<1-4-1> Over-Expression of metA Gene

To increase methionine precursor synthesis, metA gene encoding homoserine O-succinyl transferase involved in the synthesis of O-succinylhomoserine, the methionine precursor, was over-expressed.

PCR was performed by using the chromosome of E. coli w3110 as a template with primers of SEQ. ID. NO: 9 and NO: 10 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was ligated to another DNA fragment obtained from pCL1920 vector by digesting with Smal. E. coli was transformed with the ligated vector, which was then cultured in LB medium containing 50 μg/L of spectinomycin, followed by selection. The vector constructed thereby was named pMetA-CL. The schematic diagram of the pMetA-CL is shown in FIG. 3. W3-BTJ strain was transformed with the said vector. The constructed strain was named W3-BTJ/pMetA-CL and the increase of O-succinylhomoserine level therein was observed.

As another method to increase metA gene expression, metA gene was ligated to pCL1920 vector by using CJ1 promoter (CJ, Korea) and EcoRV. E. coli was transformed with the ligated vector, which was then cultured in LB medium containing 50 μg/L of spectinomycin, followed by selection. The vector constructed thereby was named pCJ-MetA-CL. W3-BTJ strain was transformed with the said vector. The constructed strain was named W3-BTJ/pCJ-MetA-CL and the increase of O-succinylhomoserine level therein was observed.

<1-4-2> Over-expression of metX Gene

To synthesize O-acetylhomoserine, metX gene encoding homoserine O-acetyl transferase involved in the synthesis of O-acetylhomoserine, the methionine precursor, was over-expressed.

PCR was performed by using the chromosome of Leptospira meyeri as a template with primers of SEQ. ID. NO: 11 and NO: 12 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.1 kbp band. The recovered DNA fragment was ligated to pCL1920 vector by using CJ1 promoter and EcoRV. E. coli was transformed with the ligated vector, which was then cultured in LB medium containing 50 μg/L of spectinomycin, followed by selection. The vector constructed thereby was named pCJ1-MetXlme-CL. W3-BTJ strain was transformed with the said vector. The constructed strain was named W3-BTJ/pCJ-MetXlme-CL and the increase of O-acetylhomoserine level therein was observed.

Another method to over-express metX gene was made by performing PCR using the chromosome of Corynebacterium as a template with primers of SEQ. ID. NO: 68 and NO: 69 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA. The recovered DNA fragment was ligated to pCL1920 vector by using CJ1 promoter and EcoRV. E. coli was transformed with the ligated vector, which was then cultured in LB medium containing 50 μg/L of spectinomycin, followed by selection. The vector constructed thereby was named pCJ-MetXcgl-CL. W3-BTJ strain was transformed with the said vector. The constructed strain was named W3-BTJ/pCJ-MetXcgl-CL and the increase of O-acetylhomoserine level therein was observed.

<1-4-3> Deletion of metA Gene

To increase the production of O-acetylhomoserine, metA gene encoding homoserine O-succinyl transferase was deleted in W3-BTJ strain. Based on the founding that only metX gene introduction resulted in the accumulation of O-succinylhomoserine, it was expected that metA gene deletion resulted in the promotion of the accumulation of O-acetylhomoserine (Table 3). To deletion metA gene, FRT one step PCR deletion was performed.

To construct metA deletion cassette, PCR was performed with primers of SEQ. ID. NO: 70 and NO: 71 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 1 minute.

The PCR product was electrophoresed on 1.0% agarose gel, followed by purification of DNA obtained from 1.2 kbp band. The recovered DNA fragment was electroporated into the E. coli W3-BTJ strain transformed with pKD46 vector. The recovered strain was streaked on LB plate medium containing chloramphenicol, followed by culture at 37° C. for overnight. Then, a strain exhibiting resistance was selected.

PCR was performed by using the selected strain as a template with primers of SEQ. ID. NO: 70 and NO: 71 under the same conditions as the above. The deletion of metA gene was identified by confirming 1.1 kb sized gene on 1.0% agarose gel. The strain was then transformed with pCP20 vector and cultured in LB medium. The final metA knock out strain was constructed in which the size of metA gene reduced to 100 kb on 1.0% agarose gel by PCR under the same conditions. Chloramphenicol marker was confirmed to be eliminated. The constructed strain was named W3-BTJA. The W3-BTJA strain was transformed with the pCJ-MeTXlme-CL vector and the resultant strain was named W3-BTJA/pCJ-MetX-CL. The strain was cultured by the same manner as described above and as a result the accumulation of O-succinylhomoserine was not observed but the production of O-acetylhomoserine was significantly, approximately 20% increased, compared with W3-BTJ.

<1-5> Conversion of L-threonine Producing Strain

Methionine precursor-producing strains were constructed by the same manner as described in Examples <1-1> to <1-3> using E. coli CJM002 (KCCM-10568), the L-threonine producing strain free from the requirement for methionine. The constructed strains were named CJM-BTJ, CJM-BTJ/pMetA-CL and CJM-BTJ/pCJ-MetA-CL, respectively. The metA gene knock-out strain was also constructed by the same manner as described in <1-4-3> using the CJM-BTJ strain and the resultant strain was named CJM-BTJA.

EXAMPLE 2 Fermentation for the Production of L-methionine Precursor

<2-1> Experiment of Flask Culture

To investigate the methionine precursor production capacity of the strain constructed in Example 1, Erlenmeyer flask culture was performed. W3-BTJ, CJM-BTJ and W3-BTJ transformed with metA and metX expression vector were cultured on LB plate media containing spectinomycin at 31° C. for overnight. A single colony was inoculated in 3 ml of LB medium containing spectinomycin, followed by culture at 31° C. for 5 hours. The culture solution was 200 fold diluted in 250 ml Erlenmeyer flask containing 25 ml of methionine precursor producing medium, followed by culture at 31° C., 200 rpm for 64 hours. HPLC was performed to compare with methionine precursor production capacity(Table 2 and Table 3). As a result, methionine production capacity was significantly increased in the methionine precursor-producing strain prepared from the L-threonine producing strain free from the requirement for methionine.

TABLE 1 Flask medium composition for methionine precursor production Concentration Composition (per liter) Glucose 40 g Ammonium sulfate 17 g KH₂PO₄ 1.0 g MgSO₄•7H₂O 0.5 g FeSO₄•7H₂O 5 mg MnSO₄•8H₂O 5 mg ZnSO₄ 5 mg Calcium carbonate 30 g Yeast extract 2 g Methionine 0.15 g Threonine 0.15 g

TABLE 2 Methionine precursor (O-succinylhomoserine) production by flask culture Glucose consumption O-succinylhomoserine OD (g/L) (g/L) W3-BTJ 10 40 0.3 W3-BTJ/pMetA-CL 12 40 1.2 W3-BTJ/pCJ-MetA-CL 12 40 1.8 CJM-BTJ 5.0 33 0.6 CJM-BTJ/pMetA-CL 6.0 36 5.2 CJM-BTJ/pCJ-MetA-CL 6.0 40 10.1

TABLE 3 Methionine precursor (O-acetylhomoserine) production by flask culture Glucose consumption O-acetylhomoserine OD (g/L) (g/L) W3-BTJ 10 40 0 W3-BTJ/pCJ-MetXlme-CL 12 40 1.5 W3-BTJ/pCJ-metXcgl-CL 12 40 1.4 W3-BTJA/pCJ-metXlme- 11 40 1.8 CL CJM-BTJ 5.0 33 0 CJM-BTJ/pCJ-metXlme-CL 5.5 40 4.8 CJM-BTJ/pCJ-MetXcgl-CL 6.0 36 4.6 CJM-BTJA/pCJ-metX-CL 5.8 40 6.5

<2-2> Large Scale Fermentation

A strain exhibiting the highest methionine precursor production capacity in Example 1 was selected to mass-produce methionine precursor, which was then cultured in a 5 L fermentor. CJM-BTJ/pCJ-metA-CL or CJM-BTJA/pCJ-metXlme-CL was inoculated in LB medium containing spectinomycin, followed by culture at 31° C. for overnight. Then, a single colony was inoculated in 10 ml LB medium containing spectinomycin, which was cultured at 31° C. for 5 hours. The culture solution was 100 fold diluted in 1000 ml Erlenmeyer flask containing 200 ml of methionine precursor seed medium, followed by culture at 31° C., 200 rpm for 3-10 hours. The culture solution was inoculated in a 5 L fermentor, followed by further culture for 50-100 hours by fed-batch fermentation. The methionine precursor concentration in the fermented solution was measured by HPLC and the results are shown in Table 5.

TABLE 4 Fermentor medium composition for methionine precursor production Composition Seed media Main media Feed media Glucose(g/L) 10.1 40 600 MgSO₄7H₂0(g/L) 0.5 4.2 Yeast extract(g/L) 10 3.2 KH₂PO₄ 3 3 8 Ammonium sulfate(g/L) 6.3 NH₄Cl(g/L) 1 NaCl(g/L) 0.5 Na₂HPO₄12H₂O(g/L) 5.07 DL-Methionine(g/L) 0.5 0.5 L-Isoleucine(g/L) 0.05 0.5 0.5 L-Threonine(g/L) 0.5 0.5

TABLE 5 Methionine precursor production in a fermentor O-succinylhomoserine O-acetylhomoserine (g/L) (g/L) CJM-BTJ/pCJ-MetA-CL >80 0 CJM-BTJA/pCJ-MetXlme- 0 >55 CL

Example 3 Production of Methionine Converting Enzyme

<3-1> Cystathionine Gamma Synthase Derived from E. coli

The metB gene encoding cystathionine gamma synthase derived from E. coli, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of E. coli as a template with primers of SEQ. ID. NO: 13 and NO: 14 as follows; 25 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NcoI/HindIII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The resultant vector was named pCJ-MetB-CL and the schematic diagram is shown in FIG. 4.E. coli W3110 was transformed with the cloned vector and then cultured on LB plate medium containing 50 μg/L of spectinomycin, followed by colony selection. The selected colony was inoculated in 3 ml of LB medium containing 50 μg/L of spectinomycin, followed by culture at 37° C. for overnight. The cultured cells were recovered, washed with 0.1 M potassium phosphate buffer (pH 7.5), suspended in 200 μL of potassium phosphate buffer, and lysed by sonication 5 times at 30 seconds intervals. The cell lysate was centrifuged at 12,000 rpm for 10 minutes and the supernatant was obtained to quantify the total protein level by using Bio-Rad protein quantification solution (BIO-Rad, USA). Protein expression was identified by SDS-PAGE. The supernatant obtained from the cell extract was used for the enzyme conversion reaction.

<3-2> O-succinylhomoserine sulfhydrylase derived from Pseudomonas sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Pseudomonas sp., which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned. As the Pseudomonas sp. microorganism, Pseudomonas aeruginosa and Pseudomonas putida were used.

PCR was performed by using the chromosome of each strain as a template with primers of SEQ. ID. NO: 15 and NO: 16 for the Pseudomonas aeruginosa and primers of SEQ. ID. NO: 17 and NO: 18 for the Pseudomonas putida as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/PacI and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <1-1> and used for the enzyme conversion reaction.

<3-3> O-acetylhomoserine Sulfhydrylase Derived from Pseudomonas sp.

The metY gene encoding O-acetylhomoserine sulfhydrylase derived from Pseudomonas sp., which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Pseudomonas aeruginosa as a template with primers of SEQ. ID. NO: 19 and NO: 20 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/PacI and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-4> Cystathionine Synthase Derived from Corynebacterium glutamicum

The metB gene encoding cystathionine synthase derived from Corynebacterium glutamicum, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Corynebacterium glutamicum as a template with primers of SEQ. ID. NO: 21 and NO: 22 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NcoIIHindIII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-5> O-acetylhomoserine Sulfhydrylase Derived from Corynebacterium glutamicum

The metZ gene encoding O-acetylhomoserine sulfhydrylase derived from Corynebacterium glutamicum, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Corynebacterium glutamicum as a template with primers of SEQ. ID. NO: 23 and NO: 24 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/Avr II and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-6> O-acetylhomoserine sulfhydrylase derived from Leptospira sp.

The metY gene encoding O-acetylhomoserine sulfhydrylase derived from Leptospira meyeri, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Leptospira meyeri as a template with primers of SEQ. ID. NO: 25 and NO: 26 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-7> O-acetylhomoserine Sulfhydrylase Derived from Saccharomyces sp.

The met25 gene encoding O-acetylhomoserine sulfhydrylase derived from Saccharomyces cerevisiae, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Saccharomyces cerevisiae as a template with primers of SEQ. ID. NO: 27 and NO: 28 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/PacI and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-8> O-succinylhomoserine sulfhydrylase derived from Chromobacterium sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Chromobacterium Violaceum, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Chromobacterium Violaceum as a template with primers of SEQ. ID. NO: 29 and NO: 30 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-9> O-succinylhomoserine Sulfhydrylase Derived from Nocardia sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Nocardia Farcinica, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Nocardia Farcinica as a template with primers of SEQ. ID. NO: 31 and NO: 32 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-10> O-succinylhomoserine Sulfhydrylase Derived from Bradyrhizobium sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Nocardia Farcinica, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Bradyrhizobium Japonicum as a template with primers of SEQ. ID. NO: 33 and NO: 34 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-11> O-succinylhomoserine Sulfhydrylase Derived from Hyphomonas sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Hyphomonas Neptunium, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Hyphomonas Neptunium as a template with primers of SEQ. ID. NO: 35 and NO: 36 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with BamHII/HindIII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-12> O-succinylhomoserine sulfhydrylase derived from Methylococcus sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Methylococcus Capsulatus, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Methylococcus Capsulatus as a template with primers of SEQ. ID. NO: 37 and NO: 38 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-13> O-succinylhomoserine Sulfhydrylase derived from Methylobacillus sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Methylobacillus Flagellatus, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Methylobacillus Flagellatus as a template with primers of SEQ. ID. NO: 39 and NO: 40 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-14> O-succinylhomoserine sulfhydrylase derived from Nitrosomonas sp.

The metZ gene encoding O-succinylhomoserine sulfhydrylase derived from Nitrosomonas Europaea, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Nitrosomonas Europaea as a template with primers of SEQ. ID. NO: 41 and NO: 42 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-15> Cystathionine Synthase Derived from Klebsiella sp.

The metB gene encoding cystathionine synthase derived from Klebsiella Pneumoniae, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Klebsiella Pneumoniae as a template with primers of SEQ. ID. NO: 43 and NO: 44 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-16> Cystathionine Synthase Derived from Bacillus sp.

The metB gene encoding cystathionine synthase derived from Bacillus Subtilis, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Bacillus Subtilis as a template with primers of SEQ. ID. NO: 45 and NO: 46 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-17> Cystathionine synthase derived from Shigella sp.

The metB gene encoding cystathionine synthase derived from Shigella flexneri 2457T, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Shigella flexneri 2457T as a template with primers of SEQ. ID. NO: 47 and NO: 48 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-18> Cystathionine synthase derived from Colwellia sp.

The metB gene encoding cystathionine synthase derived from Colwellia Psychrerythraea, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Colwellia Psychrerythraea as a template with primers of SEQ. ID. NO: 49 and NO: 50 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-19> Cystathionine Synthase Derived from Salmonella sp.

The metB gene encoding cystathionine synthase derived from Salmonella enterica serovar Paratyphi A, which would be used for the conversion of O-succinylhomoserine or O-acetylhomoserine, the methionine precursor, into methionine, was cloned.

PCR was performed by using the chromosome of Salmonella enterica serovar Paratyphi A as a template with primers of SEQ. ID. NO: 51 and NO: 52 as follows; 30 cycles of denaturation at 94° C. for 30 seconds, annealing at 55° C. for 30 seconds, extension at 72° C. for 2 minutes.

The obtained DNA fragment was digested with NdeI/AvrII and cloned into pCL-CJ1 vector (CJ, Korea) digested with the same enzymes. The supernatant of cell extract was obtained using the cloned vector by the same manner as described in Example <3-1> and used for the enzyme conversion reaction.

<3-20> Comparison of the Activities of the Converting Enzymes using OSHS as a Substrate

The activity of each enzyme solution obtained in Examples <3-1> to <3-19> was compared to select the optimum methionine converting enzyme.

First, O-succinylhomoserine (Sigma, USA) was dissolved in 0.1 M potassium phosphate buffer (pH 7.5) with the concentration of 3 mM. Pyridoxal 5′-phosphate (Sigma, USA) used as a coenzyme was added into the reaction solution with the final concentration of 10 μM. Methylmercaptan (Methylmercaptan, Tokyo Kasei Organic Chemicals, Japan) used as another substrate was added into the reaction solution with the final concentration of 2 mM. 1 ml of the reaction solution was placed in 37° C., to which 10 μL of each enzyme solution (protein conc.: 5 mg/ml) was added. 100 μL of the reaction solution was collected every 5-10 minutes and added into 900 μL of 4 mg/ml DTNB (Sigma, USA) solution. OD415 was measured to confirm the on-going of the reaction.

DTNB was reacted with SH group of methylmercaptan remaining in the reaction solution and thus synthesized a yellow substance. Thus, whether the reaction was going on or not was checked by observing the disappearance of yellow color of the reaction solution resulted from the conversion reaction of methylmercaptan into methionine.

As shown in FIG. 5, O-succinylhomoserine sulfhydrylase derived from Chromobacterium sp., O-succinylhomoserine sulfhydrylase derived from Nocardia sp., O-succinylhomoserine sulfhydrylase and O-acetylhomoserine sulfhydrylase derived from Pseudomonas sp. were shown to have high enzyme activities. Other enzymes also showed some degree of activity but their reaction speeds were relatively slow. Reactivity to the substrate of each enzyme was summarized in Table 6. Upon completion of one-hour reaction, HPLC was performed to confirm the final productions of methionine and succinic acid. The results are shown in Table 7.

<3-21> Comparison of the Activities of the Converting Enzymes using OAHS as a Substrate

Experiment was performed with O-acetylhomoserine by the same manner as described in Example <3-20>. O-acetylhomoserine was purified from the supernatant of fermented solution. The same reaction solutions and enzyme solutions as used for the experiment with O-succinylhomoserine were used for the reaction. As shown in FIG. 6, O-succinylhomoserine sulfhydrylase derived from Hyphomonas sp., O-acetylhomoserine sulfhydrylase derived from Pseudomonas sp., O-succinylhomoserine sulfhydrylase derived from Chromobacterium sp. and O-acetylhomoserine sulfhydrylase derived from Leptospira sp. were shown to have high enzyme activities. Other enzymes also showed some degree of activity but their reaction speeds were relatively slow. Reactivity to the substrate of each enzyme was summarized in Table 6. Upon completion of one-hour reaction, HPLC was performed to confirm the final productions of methionine and succinic acid. The results are shown in Table 8.

TABLE 6 Conversion reaction of O-succinylhomoserine and O-acetylhomoserine by the enzyme derived from each strain Substrate Strain No. Gene specificity Strain (ATCC) (KEGG) OSH OAH Escherichia Coli K12 55151 MetB + + Pseudomonas aurogenosa 17933 MetZ +++ + MetY ++++ ++++ Pseudomonas putida 17390 MetZ ++++ + Corynebacterium glutamicum 13032 MetB + + MetY + + Leptospira meyeri 43278 MetY + ++ Saccharomyces cerevisiae 2704 Met25 + + Chromobacterium Violaceum 12472 MetZ ++++ +++ Nocardia Farcinica 3318 MetZ ++++ + Bradyrhizobium Japonicum 10324 MetZ + + Hyphomonas Neptunium 49408 MetZ + ++++ Methylococcus Capsulatus 19069D-5 MetZ + + Methylobacillus Flagellatus 51484D MetZ + + Nitrosomonas Europaea 19718D MetZ + + Klebsiella Pneumoniae 25955 MetB + + Bacillus Subtilis 10783 MetB + + Shigella flexneri 2457T 700930D-5 MetB + + Colwellia Psychrerythraea BAA-618D MetB + + Salmonella enterica 9150D MetB + + serovar Paratyphi A

TABLE 7 Production capacity of methionine and succinic acid from O- succinylhomoserine by each enzyme amount of amount of Methionine Succinic acid Enzyme gene (g/L) (g/L) Corynebacterium glutamicum metB 0.05 0.03 Escherichia Coli metB 0.14 0.1 Nocardia Farcinica metZ 0.21 0.17 Pseudomonas putida metZ 0.22 0.17 Pseudomonas aurogenosa metZ 0.22 0.17 Chromobacterium Violaceum 0.22 0.17 Pseudomonas aurogenosa metY 0.21 0.17

TABLE 8 Production of methionine and acetic acid from O-acetylhomoserine by each enzyme Amount of amount of Methionine Acetic acid Enzyme gene (g/L) (g/L) Pseudomonas aurogenosa metY 0.22 0.081 Chromobacterium Violaceum metZ 0.18 0.068 Hyphomonas Neptunium metZ 0.22 0.082 Corynebacterium glutamicum metY 0.05 0.015 Leptospira meyeri metY 0.15 0.05

<1-22> Identification of Feed-Back Inhibition for Converting Enzyme

Feed-back inhibition in the presence or absence of methionine was identified by the same manner as described in Examples <3-20> and <3-21>. The reaction solutions were prepared by the same manner above and the same reaction was performed by adding or not adding 5 g/L of methionine in the each reaction solution. The reaction speed in the reaction solution without methionine was regarded as 100%, based on which the remaining activity in the presence of methionine was calculated as %. The results are shown in table 9.

As a result, the activity of each O-acetylhomoserine sulfhydrylase derived from Pseudomonas sp., O-succinylhomoserine sulfhydrylase derived from Nocardia sp. and O-acetylhomoserine sulfhydrylase derived from Leptospira sp. was inhibited by methionine, suggesting that those enzyme activities were inhibited by feed-back system in the presence of methionine. Enzymes without feed-back system were used for further reactions. It was presumed that the enzyme was inhibited by feed-back system in the above embodiment to be used in the same reaction where a mutant strain free from feed-back system was used.

TABLE 9 Inhibition of enzyme activity by methionine Remaining activity (%) Enzyme gene OSHS OAHS Chromobacterium Violaceum metZ 97 100 Pseudomonas aurogenosa metY 54 53 Nocardia Farcinica metZ 68 Pseudomonas putida metZ 98 Pseudomonas aurogenosa metZ 98 Leptospira meyeri metY 45 Hyphomonas Neptunium metZ 100

<1-23> Comparison of Homology Among the Converting Enzymes

Homology among the converting enzymes used for the conversion reaction was compared to investigate the interactions of the reactivity to O-succinylhomoserine and O-acetylhomoserine and the feed-back inhibition.

From the comparison of homology among the converting enzymes used herein, it was confirmed that the homology between metZs encoding O-succinylhomoserine sulfhydrylase and the homology between metYs encoding O-acetylhomoserine sulfhydrylase were higher than the homology between metZs and metYs. In connection with the above embodiment, there are many case of the metZ encoding O-succinylhomoserine sulfhydrylase which does not exhibit the feed-back inhibition. However, it was identified that the metY encoding O-acetylhomoserine sulfhydrylase was inhibited by relatively high feed-back system because all the enzymes used in the examples were inhibited by feedback. Regarding the selectivity to O-succinylhomoserine and O-acetylhomoserine, the metZ gene group exhibited high selectivity to O-succinylhomoserine, while the metY gene group exhibited high selectivity to O-acetylhomoserine. In the meantime, metY derived from Pseudomonas putida and metZ derived from Chromobacterium Violaceum exhibited specifically high reactivity to both substrates.

The amino acid sequences of all the enzymes used herein were aligned by Clustal W program (DNAstar). As a result, they all have the domains represented by the following sequences. Therefore, the enzymes that have the following domains can produce methionine by the same manner.

Domain 1: (SEQ ID NO: 72) Y-(S, I, T, V)-R-X-X-(N, S) Domain 2: (SEQ ID NO: 73) (V, A, I)-(V, L, I)-D-N-X-(F, V, M, I)-X-(T, S)- (P, A)-X-(L, I)-(Q, C, V)-X-(P, G)-(L, F)-X-(L, M, H)-G-(A, V)-(D, H) Domain 3: (SEQ ID NO: 74) (S ,A, G, P)-(P, A ,V)-F-(N, D)-(A, S)-(W, F, Y)- X-X-X-(K, Q, R, S)-G-(L, M, V, I, M)-(E, K, D, R)-T-(L, M)- Domain 5: (SEQ ID NO: 75) (H, Y)-(P, A)-(A, S)-(T, S)-(T, M, Q)-(T, S)-H Domain 6: (SEQ ID NO: 76) (V, I, L)-R-(V, I, L, F)-(S, A)-(V, I, T)-G-(L, I)-E

Example 4 Methionine Conversion Reaction by Methionine Converting Enzyme

<4-1> Mass-Production of Converting Enzyme

To mass-produce the strain of producing methionine converting enzyme constructed in Example 2 (2-2 and 2-8), the strain was cultured in a 1 L fermentor. A strain(W3110) was transformed with metZ expression vector derived from Pseudomonas sp. or metZ expression vector derived from Hyphomonas sp. The transformed strains were inoculated on LB plate medium containing spectinomycin, followed by culture at 30-40° C. for overnight. The obtained single colony was inoculated in 40 ml of LB medium containing spectinomycin, followed by culture at 30-40° C. for 5 hours. The cultured metZ expressing strain derived from Pseudomonas sp. and metZ expressing strain derived from Hyphomonas sp. were cultured in a 1 L fermentor at 30-40° C., 600-900 rpm for 15-30 hours. The composition of the medium for the culture is shown in Table 10.

The methionine converting enzyme solution was prepared by homogenizing cells of the fermented solution using sonication.

TABLE 10 Composition of the medium for the production of converting enzyme 2XYT medium composition Yeast extract (g/L) 10 Tryptophan (g/L) 16 Glucose (g/L) 40 Spectinomycin (g/L) 50

<4-2> Methionine Conversion Reaction

Methionine conversion reaction was performed by using O-succinylhomoserine converting enzyme solution derived from Pseudomonas sp. and O-acetylhomoserine converting enzyme solution derived from Hyphomonas sp. prepared in Example 4 (4-1) respectively in the fermentation solution of O-succinylhomoserine and O-acetylhomoserine prepared in Example 2 (2-2).

0.1 L of cell-lysed enzyme culture solution was added to 2.0 L of fermentation solution of methionine precursor which did not remove the cell, to which 0.3 L of 15% Na-methylmercaptan was added to initiate the reaction. Two hours later, the fermentation solution was recovered and cells were removed. HPLC was performed to confirm the methionine production. The results are shown in Table 11.

TABLE 11 L-methionine Succinic acid Acetic acid (g/L) (g/L) (g/L) Fermentation solution of O- >42 >33 0 succinylhomoserine (>80 g/L) Fermentation solution of O- >40 0 >15 acetylhomoserine (>55 g/L)

As a result, while L-methionine was produced with low concentration of up to 10 g/L in the conventional method, L-methionine could be mass-produced by the method of the present invention at the concentration of up to 30 g/L.

INDUSTRIAL APPLICABILITY

The method of the invention enables the selective production of L-methionine, which is superior to the conventional chemical synthesis producing D-methionine and L-methionine together, and the production of organic acid such as succinic acid or acetic acid as a by-product without additional independent processes. 

What is claimed is:
 1. A method for producing L-methionine, comprising: i) culturing an L-methionine precursor-producing microorganism strain in a fermentation solution, so that the L-methionine precursor accumulates in the fermentation solution; and ii) mixing a converting enzyme and methylmercaptan or its salts with at least a portion of the fermentation solution to convert the accumulated L-methionine precursor into L-methionine.
 2. The method of claim 1, wherein said the L-methionine precursor is an O-acylhomoserine.
 3. The method of claim 1, wherein the L-methionine precursor-producing microorganism strain of step i) is selected from the group consisting of Escherichia sp., Erwinia sp., Serratia sp., Corynebacterium sp., Pseudomonas sp., Salmonellar sp., Brevibacterium sp., and yeasts.
 4. The method of claim 3, wherein the L-methionine precursor-producing microorganism strain is selected from Escherichia sp. and Corynebacterium sp.
 5. The method of claim 4, wherein the L-methionine precursor-producing microorganism strain is Escherichia coli.
 6. The method of claim 4, wherein the L-methionine precursor-producing microorganism strain is Corynebacterium glutamicum.
 7. The method of claim 1, wherein said at least a portion of the fermentation solution is a substantially cell-free solution.
 8. The method according to claim 1, wherein the converting enzyme is provided as a cell-free extract of a microbial culture.
 9. The method according to claim 1, wherein the converting enzyme is a recombinant enzyme.
 10. A method for producing L-methionine and organic acid, which comprises the following steps: i) producing L-methionine precursor in a fermentation medium by the fermentation of an L-methionine precursor-producing microbial strain, wherein the L-methionine precursor-producing microbial strain is prepared by deleting the activity of cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase involved in the degradation of L-methionine precursor and by enhancing the activity of homoserine O-succinyl transferase or homoserine O-acetyl transferase involved in L-methionine precursor synthesis from homoserine; and ii) producing L-methionine and organic acid outside of the L-methionine precursor-producing microbial strain by the enzyme reaction using the L-methionine precursor and methylmercaptan or its salts as substrates, wherein the enzyme reaction is performed by an enzyme not present in said precursor-producing strain.
 11. The method of claim 10, wherein the L-methionine precursor-producing microbial strain is selected from the group consisting of Escherichia sp. and Corynebacterium sp.
 12. The method of claim 10, wherein the cystathionine gamma synthase or O-succinylhomoserine sulfhydrylase or O-acetylhomoserine sulfhydrylase of step i) is encoded, respectively, by metB, metZ, and metY, or functional mutants thereof.
 13. The method of claim 10, wherein the L-methionine precursor-producing microbial strain is a strain whose threonine, isoleucine or lysine biosynthesis pathway is weakened or deleted.
 14. The method of claim 10, wherein the enzyme of step ii) is from a strain selected from the group consisting of Escherichia sp., Erwinia sp., Serratia sp., Providenciasp., Corynebacterium sp., Pseudomonas sp., Leptospira sp., Salmonellar sp., Brevibacterium sp., Hyphomonas sp., Chromobacterium sp., Nocardia sp., and yeasts.
 15. The method of claim 10, wherein the enzyme of step ii) is from a strain selected from the group consisting of: Escherichia Coli, Pseudomonas aurogenosa, Pseudomonas putida, Corynebacteria glutamicum, Leptospira meyeri, Saccharomyces cerevisiae, Chromobacterium Violaceum, Nocardia Farcinica, Bradyrhizobium Japonicum, Hyphomonas Neptunium, Methylococcus Capsulatus, Methylobacillus Flagellatus, Nitrosomonas Europaea, Klebsiella Pneumoniae, Bacillus Subtilis, Shigella flexneri, Colwellia Psychrerythraea, and Salmonella enterica serovar Paratyphi A.
 17. A method for producing L-methionine, comprising: i) separating fermentation solution from a culture of an L-methionine precursor-producing microorganism strain to produce a supernatant; and ii) contacting a cell-free extract from a second culture with at least a portion of said supernatant in the presence of methylmercaptan or its salts to convert L-methionine precursor in said supernatant into L-methionine
 18. The method of claim 17, wherein the supernatant contains O-acylhomoserine.
 19. The method according to claim 17, wherein the second culture is selected from the group consisting of Escherichia sp., Erwinia sp., Serratia sp., Providencia sp., Corynebacterium sp., Pseudomonas sp., Leptospira sp., Salmonellar sp., Brevibacterium sp., Hyphomonas sp., Chromobacterium sp. and Nocardia sp., and yeasts.
 20. The method of claim 17, wherein the second culture is selected from the group consisting of Escherichia Coli, Pseudomonas aurogenosa, Pseudomonas putida, Corynebacteria glutamicum, Leptospira meyeri, Saccharomyces cerevisiae, Chromobacterium Violaceum, Nocardia Farcinica, Bradyrhizobium Japonicum, Hyphomonas Neptunium, Methylococcus Capsulatus, Methylobacillus Flagellatus, Nitrosomonas Europaea, Klebsiella Pneumoniae, Bacillus Subtilis, and Shigella flexneri. 